Hydra are truly fascinating small
aquatic animals. Most hydra are tiny, reaching a maximum of only about 30 mm long when
fully extended. They are barely visible to the naked eye and a hand lens or microscope are
needed to be able to see them properly. When the body is extended with the tentacles
waving in the water, they resemble a small piece of string which has become frayed at one
end. If disturbed, a hydra will contract its body and tentacles, so that it now resembles
a small 'blob'.

They can be found in freshwater ponds and slow-moving
rivers, where they usually attach themselves to submerged plants or rocks. The hydra
pictured above came from pond water which contained large quantities of filamentous algae
(a very simple kind of plant).

There are four species of *Hydra in Britain. They range in colour from green, through varying shades of
brown. The green hydra above (Chlorohydra
viridissima) gets its colour from green algae which live
inside its tissues in a mutually beneficial relationship.

The algae living inside the hydra benefit from having a
sheltered safe environment and obtain food by-products from the hydra. The hydra also
benefit from algal products. It has been shown that hydra which are kept in the light, but
otherwise starved, survive better than hydras without any green algae inside them. They
are also able to survive in water with a low dissolved oxygen concentration because the
algae supply them with oxygen. This oxygen is a by product of the photosynthesis carried
out by the algae. Green hydra transmit the algae from one generation to the next in the
eggs.

Hydras are fairly simple animals in terms of their body
construction..

Their bodies are basically just a hollow
cylinder, which is solid at the base, with an opening at the mouth end. The mouth leads
into the hollow interior of the cylinder which acts as the digestive cavity. The mouth is
surrounded by a variable number of tentacles (usually somewhere between 6 and 10). Because
the digestive cavity has only one opening, food and subsequently, digested waste, both
come in and go out through the same opening.

A structure called a basal disc on the bottom of the tube-like body
secretes a sticky substance which helps hydras to attach themselves to a solid base.

(left - a diagrammatic
section through a hydra)

Hydras move their bodies about in the water while they
are attached, extending and contracting by a mixture of muscle movement and water
(hydraulic) pressure. This hydraulic pressure is created inside their digestive cavity.
The cells lining the digestive cavity have little flagellae (minute hair-like structures)
on their surface. By beating these flagellae like miniature oars, currents can be created
which draw water into the digestive cavity. This raises the water pressure inside, which
causes the hydra's body to increase in length. The body of a single hydra can change from
being 20mm long when it is relaxed and extended, to being as little 0.5mm long when it has
contracted (using muscular movement) after being disturbed.

Hydra are not always attached to the substrate and can
move from one spot to another, either by gliding along on the basal disc or by
somersaulting along. When somersaulting, they detach the basal disc and then bend over and
place the tentacles down on the substrate. This is followed by reattaching the basal disc
further along, before repeating the whole process again. They may also float about in the
water upside down. When they are floating, it is because the basal disc produces a gas
bubble which carries the animal up to the water surface.

The single-celled algae (Chlorella) which give Chlorohydra its green colour,
live inside the cells lining the digestive cavity. Interestingly, these algae can also be
found living freely outside of hydras.

One side of tree trunks (often the north side) in woods
is sometimes covered in a smooth green coating almost as if it has been painted. This
green covering usually advertises the presence of a whole mass of free-living Chlorella algae (although other
species of green algae will also grow on tree trunks).

Hydras are close relations of sea anemones (left), which can often be found
in rock pools on seashores. Like the hydra, they have a barrel-shaped body, topped by a
mouth surrounded by tentacles.

In contrast to sea anemones,
most hydras live in freshwater, although one colonial species found in Britain (Cordylophora lacustris) has
spread into freshwater habitats from brackish water (water which is partly salty).

Hydra are carnivorous and feed mainly on small
crustaceans like water fleas (Daphnia) and small worms. Although hydra are fairly simple animals, the stinging
cells which they use to catch their prey are quite complex structures.

They have a number of different types of stinging cells, called nematocysts, on their
tentacles. Coiled tubes are shot out at any passing prey which has had the misfortune to
touch a tentacle. The prey gets all tangled up and unable to move. A second kind of
nematocyst, which may have barbs or spines on it, is also fired into the prey. This
releases toxins into the prey which paralyse it.

This toxin is too weak to have any effect on humans
which happen to touch the tentacles, unlike the toxins from the stinging cells of
jellyfish, which can cause painful stings to humans. (Jellyfish are also closely related
to Hydra.) Once the
prey has been paralysed, the tentacles will pull it towards the mouth, where it will be
swallowed to be digested in the digestive cavity. Much of the digestion is accomplished by
enzymes secreted into the cavity. This means that hydra can eat relatively large prey in
comparison to their size. Small particles resulting from digestion are then engulfed by
the cells lining the digestive cavity.

In catching prey, hydra use up a lot of their
nematocysts, which can only be discharged once. A study was carried out on one species of Hydra which showed that 25% of
the nematocysts on the tentacles were used up in the process of catching and eating one
brine shrimp. It took about 48 hours for new nematocysts to be produced to replace the
used up ones.

Hydra reproduce in two ways
depending on the season.

The first, asexual method, involves
budding new individuals from the body wall. Basically, a branch of the body cylinder is
produced, with a digestive cavity, which is continuous with that of the parent, tentacles
and a mouth at the top.

Eventually the base will pinch off from the parent, becoming a new
separate hydra. This is an asexual process because no fusion of sexual cells is involved
and new individuals produced in this way are genetically exactly the same as their parent.

This is the usual method of reproduction in warmer months.

As it begins to get colder, sexual reproduction may
start to take over. Most Hydra species have individuals which are either male or female. Eggs are produced
in the outer body wall of female hydra and are fertilised by sperm released into the water
by neighbouring male hydras. Some species of Hydra are hermaphrodite, with each individual having both male and female
reproductive organs.

The seasonal timing of sexual reproduction is a way of
helping the species survive over the winter. This is because the fertilised eggs have a
tough shell or covering and can form a resting stage over cold periods. In the spring, the
shell softens and a new hydra will emerge.